Cameron Reed estimates the yield of a fission bomb on page 105 of the February 2018 issue of the American Journal of Physics. An article on page 119 of the same issue compares theory and experiment for fluid sloshing inside a partly filled bottle rolling down a slope, or along the floor and bouncing off a wall. The March 2018 issue has an article on page 169 that explains why a line of color (green if held horizontally) appears diametrically across a CD exposed to white light from the side. Another article on page 206 investigates in detail how a pot-in-pot cooler works. Finally, Dean Pesnell on page 338 of the May 2018 issue considers what the trajectory of a cannonball launched horizontally off a tall mountain would be if the the ball could freely pass through the earth treated as a sphere of uniform mass density.

An article on page 149 of the March 2018 issue of The Physics Teacher presents data showing that there is a correlation between hours of sleep the night before a final exam and score on that exam. Joe Amato explains how the range of an ICBM can be estimated from its observed maximum altitude on page 210 of the April 2018 issue. The same issue on page 222 discusses a video of the motion of a two-ball Newton's cradle where the mass ratio of the two balls is 3:1. Also see if you can deduce the simple rule for the dice game "Petals Around a Rose" discussed on page 262; you can test your prediction online. An article on page 317 of the May 2018 issue puts a drone quadcopter in vacuum to determine whether the change in pointing direction is due to conservation of angular momentum or atmospheric drag when the speed of the blades is changed. The iPhysicsLabs column on page 324 of the same issue measures the drag coefficient of a car, bus, fire engine, and cyclist.

Article 024003 in the March 2018 issue of Physics Education rebuts many of the common concerns about nuclear energy. Discussion of cameras and example ideas of slow-time-lapse recordings (over a period of hours or days) for physics analysis is found in articles 035019 and 035030 of the May 2018 issue. A Lagrangian-based model of the pressure and density in a sonoluminescent bubble is presented in article 025807 of the March 2018 issue of the European Journal of Physics. Both journals can be found online.

Deepak Dhar on page 183 of the February 2018 issue of Resonance proposes that mathematics is invented not discovered, and hence a number such as π did not exist thousands of years ago. Also an article on the Maragoni effect in fluid mechanics on page 225 of the same issue has links to wonderful videos of effects driven by surface tension gradients. Another article on fluids with links to lovely videos is on page 491 of the April 2018 issue presenting the behavior of air bubbles in liquids. These articles can be freely accessed online.

Cyclic voltammetry is reviewed for beginners on page 197 of the February 2018 issue of the Journal of Chemical Education. An interesting study on page 347 of the March 2018 issue investigates what kinds of textbook illustrations best promote student learning.

Page 460 of the November 2017 issue of The Physics Teacher has a nifty article explaining how to construct a boat propelled by a magnetohydrodynamic drive, like in the movie "The Hunt for Red October." The demonstration highlights many of the downsides of such a propulsion system compared to what the movie portrays. On page 588 of the December 2017 issue, James Lincoln convincingly argues that slow-motion video greatly improves the pedagogy of many common physics demos. Finally the January 2018 issue has a variety of short articles that analyze the physics behind interesting effects such as an ice-hockey slapshot on page 7, the tendency for dirt to accumulate along edges of melting snow on page 10, optical deflection of light by prisms on pages 14 and 18, and properties of homopolar motors and generators on pages 47 and 61.

Romanelli proposes a Stirling cycle based on a polytropic (intermediate between an isothermal and an adiabatic) process on page 926 of the December 2017 issue of the American Journal of Physics. McCreery and Greenside insightfully analyze the electric field of a uniformly charged cubic shell on page 36 of the January 2018, as a contrast to the familiar textbook examples of spherical and cylindrical shells for which the internal field is zero.

Article 065010 in the November 2017 issue of Physics Education experimentally investigates why a cup filled partly or completely with liquid, covered with a sheet of paper, and inverted does not necessarily spill its contents. The surprising demonstration of disassembling a charged capacitor consisting of a sheet of glass between two metal plates, handling the separated parts, and then reassembling the capacitor and finding it is still charged is discussed in article 065202 of the November 2017 issue of the European Journal of Physics. Article 065204 in the same issue investigates why fluorescent tubes begin to flicker before they burn out. Article 015002 in the January 2018 issue explains why Newton's bucket cannot be used to determine earth's rotation, the problem being that the necessarily finite size of the bucket means that earth's gravity will be nonuniform over the surface of the liquid in the bucket. Other papers that caught my eye in the same issue are Hecht's discussion of the arrow of time in article 015801, and measurement of the sodium doublet with a Michelson interferometer in article 015704. Both journals can be found online.

The November 2017 issue of Resonance has an article on page 1061 reviewing the properties of perovskite solar cells, a topic of current industrial interest. A spin coater in a dry box is mostly all one needs to prepare such devices, thereby making this research field accessible to undergraduates. In the same issue, a paper on page 1085 explains how the Rayleigh-Taylor instability is responsible for many of the rolling swirls seen in clouds, nebulae, and similar gas interfacial dynamics.

The November 2017 issue of the Journal of Chemical Education is devoted to polymers, with a wealth of articles about their properties, syntheses, and classroom experiments & demonstrations.

Article 020124 in Physical Review Physics Education Research at finds a significant correlation between personality types of students (such as their Myers-Briggs temperament) and their performance on the Force Concept Inventory which tests understanding of concepts in basic Newtonian mechanics. This study collected data measuring such correlations, but does not attempt to explain the cause of these correlations, nor how classroom pedagogy could be modified to address them.

On page 293 of the May 2017 issue of The Physics Teacher a trio of Japanese educators propose an improved motor design consisting of coiled copper wire suspended on clips above a permanent magnet. Rather than the conventional design which uses a single coil and thus requires a commutator (by stripping the insulation off only one side of the copper wire) and priming (by giving the coil an initial twist to start it), the new design uses a figure-8 pair of coils to avoid those two issues and to increase the energy conversion efficiency. The September 2017 issue has a large number of thought-provoking articles about the issue of racial diversity in physics education.

Pantaleone analyzes the wondrous chain foundation on page 414 of the June 2017 issue of the American Journal of Physics. His key idea is that as a link is pulled up at an angle from the pile, there is an upward reaction force from the pile on that link. Digilov uses the Lambert W function to analyze the time-dependent weight of a vessel from which liquid is draining out through a capillary tube on page 510 of the July issue. On page 522 of the same issue, three Brazilian physicists present an undergraduate experiment to measure thermal lensing of a Gaussian laser beam in soy sauce. A group at Smith College notes on page 663 in the September issue that the gravitational self-interaction of earth’s tidal and terrestial bulges produce a substantial correction to the conventional values; a helpful analogy is drawn to the solution of Laplace’s equation for a charged shell.

Article 043004 in the July 2017 issue of Physics Education uses a unipolar motor to demonstrate angular momentum conservation; article 043006 emphasizes that the stopping potential in the photoelectric effect determines the work function of the collector and not of the emitter; and article 045021 discusses the theoretical upper limit on the possible mass of stars. Article 045402 in the July 2017 issue of the European Journal of Physics revisits the issue of the difference between the standard expressions for the phase velocity of a free matter wave in the relativistic and classical limits, and why both disagree with the particle velocity. Article 055202 in the September issue discusses the problem of determining the static charge distribution along a finite straight wire; I was surprised to learn this simple configuration is unsolved and possibly indeterminate. Both journals can be found online.

The June 2017 issue of Resonance has an article about radio-frequency identification tags and another about orbital precession due to the general theory of relativity. The July issue has a paper about using interferometry to measure the diameter of stars, as first performed by Michelson. The August issue has a historical review of density functional theory. Finally the September issue discusses some experiments with antibubbles, which are spherical shells of air floating in a soap solution. These articles can be freely accessed online.

A Python program to solve Schrödinger’s equation is presented on page 813 of the June 2017 issue of the Journal of Chemical Education. An editorial on page 825 of the July issue discusses two recent studies which show no evidence that instruction tailored to particular student learning styles results in improved achievement; an article on page 976 of the same issue explains how to construct the periodic table step by step based on atomic orbitals. Synthesis and characterization of carbon and of perovskite quantum dots are respectively discussed on pages 1143 and 1150 of the August issue. The journal archives are online.

Article 010130 in Physical Review Physics Education Research at discusses the pedagogical implications of the distinction between the gravitational definition of weight (as the net gravitational force acting on an object) and the operational definition (as the contact force measured by a scale). Article 020110 considers student views about the nature and process of experimental physics compared to those of practicing physicists.

An article published in the Journal of Modern Optics shows that the Doppler shift in the emission frequency of a moving atom gives rise to a velocity-dependent frictional force, in apparent contradiction to relativity. (The velocity of the atom and hence the force depends on the motion of the observer.) This paradox is explained by the change in momentum due to the relativistic loss in mass of the atom when it radiates away a photon, making for an alternative method of deducing Einstein’s mass-energy relation.

The January 2017 issue of The Physics Teacher has articles on page 6 about how to construct an acoustic levitation apparatus and on page 8 about what a dolphin would see when it looks up toward the surface of its tank. Page 83 of the February issue uses a cell phone to measure the electromagnetic skin depth of salt water. A brief article on page 236 of the April issue shows that Newton's third law can be "derived" from the second law by assuming that the mass of a composite system equals the sum of the masses of its component objects. Finally, a fascinating study on page 268 of the May issue explores whether the Hindenburg disaster is mostly due to an explosion of the hydrogen gas in the dirigible or to the flammability of the coating applied to the airship's fabric.

Page 98 of the February 2017 issue of the American Journal of Physics compares theory and experiment for the weight of an hourglass containing flowing sand. Another article on page 124 of the same issue shows experimentally that for the familiar problem of a ball starting on top of a hemisphere and rolling down it, the ball begins to slip before it loses contact with the surface. An interesting note on page 228 of the March issue proposes using a gyroscope mounted on a boat in a small pool to demonstrate how a gyrocompass works.

Article 035006 in the May 2017 issue of Physics Education quantifies experimentally the force required to pull apart two interleaved books as a function of number of pages and distance of page overlap. Article 025204 in the March 2017 issue of European Journal of Physics catalogs the general motions of a charged particle moving in crossed uniform electric and magnetic fields. Both journals can be accessed online.

Interesting articles in the January and February 2017 issues of Resonance include "Endangered Elements of the Periodic Table" and the two-part "How Do Wings Generate Lift?"

Atomic weights are not constants of nature for reasons discussed on page 311 of the March 2017 issue of the Journal of Chemical Education. Page 320 of the same issue discusses the first manmade element, technetium, in terms of the nuclear shell model.

Article 010110 in Physical Review Physics Education Research investigates the question of whether including diagrams in problem statements on tests improves student performance and learning? The answer is often no.

If you give multiple-choice tests, you may be interested in Heidi Wainscott’s analysis in the November 2016 issue of The Physics Teacher of the effect when students change their answers during an exam. In the same issue, the rotation of a can suspended near a jumping ring apparatus is a novel “twist” on this familiar demonstration. Another interesting surprise is Yaguo Ogawara’s proof that one gets improved traction by driving a front-wheel car backward up an icy slope than by driving forward. Finally, Hewitt’s Figuring Physics column concerns the difference in force when punching a heavyweight versus a lightweight boxer.

Two intriguing articles involve entropy in the January 2017 issue of the American Journal of Physics. The earth receives low-entropy energy from the sun and exhausts high-entropy energy to space; a paper on page 14 turns things around and imagines receiving energy from the cosmic background and exhausting it to a black hole instead. Then an article on page 23 devises a reversible way to exchange the temperatures of a hot and a cold body by splitting each of them into infinitesimal pieces and sequentially bringing those pieces into thermal contact.

The Indian Academy of Sciences journal Resonance often has useful review articles about topics in science and mathematics. Ones that caught my eye recently include a discussion of phase transitions in terms of the Ising model in the October 2016 issue, the temperature of gas in the interstellar medium in the November issue, and the inequalities among the arithmetic, geometric, and harmonic means (with applications and problems) in the December issue.

An article on page 1961 of the November 2016 issue of the Journal of Chemical Education presents an experimental method to measure the speed of sound in various gases. The Fourier transform is taken of the sound recorded while white noise is generated in an acoustic tube. The white noise is created by the expansion of gas rushing into the evacuated tube.

A wonderful comparison of pinhole images of the sun projected onto a wall by openings in a tree before and during a partial solar eclipse can be found on page 259 of the May 2016 issue of The Physics Teacher. Another lovely photograph in the same issue is of a chain fountain on page 320. David Keeports presents some optics-based tricks to correct near- or far-sightedness without glasses on page 375 of the September 2016 issue.

In the June 2016 issue of the American Journal of Physics, John Lekner works out the charge ratio as a function of distance of separation of two like-charged (but different radius) metal spheres that will cause them to attract (rather than repel) due to their mutual polarization. An article of page 413 of the same issue makes the provocative claim that many counterclockwise thermodynamic cycles are not refrigerators. A paper by Robert Hilborn in the July 2016 issue shows that glib statements in an introductory course about electromagnetic field energy being the mechanism for exchange between charges are seldom helpful and often flat out wrong; it is preferable to stick instead to the potential energy of the configuration. A systematic approach to the problem of determining the general shapes of noncircular wheels rolling smoothly on nonflat roads is presented on page 581 of the August 2016 issue.

Article 045002 in the July 2016 issue of the European Journal of Physics hypothesizes that the reason a cat can survive falls from small or large heights but not from intermediate values (corresponding to about the seventh floor of a building) is because the jerk has a maximum for a descent of 20 m, causing a cat to stiffen with fear. In the May 2016 issue of Physics Education, de Carvalho considers some contradictions in the force and torque balance of a two-pan scale. In the July 2016 issue, Marciotto calls for an improved derivation of Bernoulli’s principle in the introductory course. Both journals can be accessed online.

Page 429 of the May 2016 issue of Resonance has an article discussing some errors in the Feynman Lectures on Physics about crystal symmetries. Page 447 of the same issue considers the paradox that, due to the shell theorem, there would be no gravitational force in an infinite homogenous universe. Finally page 453 shows that a cube of heavy ice would sink in ordinary water, unlike what a cube of ordinary ice does; dying the cubes with food coloring makes for a nifty demo. A special issue in the June 2016 issue on Hamilton reviews his work on optical wavefronts on page 511 and on quaternions on page 529.

An article on page 1289 of the July 2016 issue of the Journal of Chemical Education discusses a detailed lab to investigate the bandgap, doping, and structure of light-emitting diodes. Pages 1340 to 1352 of the August 2016 issue has a great set of science book reviews by four different readers, including “What Every Science Student Should Know” and “Seven Brief Lessons on Physics.” Pages 1441 to 1451 of the same issue describes two (admittedly lengthy) experiments to prepare and characterize nanoparticle-based materials. Finally the September 2016 issue has an article on page 1578 applying the variational method to calculating bonding and antibonding orbitals of hydrogen molecules starting from Gaussian trial wavefunctions.

Article 010135 in Physical Review Physics Education Research compares three different approaches to teaching wave optics to introductory students: by sketching sinusoidal waves, via snapshots of the electric field at various instants, or by using phasors.

In the February 2016 issue of The Physics Teacher, Prentis and Obsniuk present an introductory-level discussion of how the Helmholtz free energy can be thought of as a competition between minimizing energy and maximizing entropy at equilibrium, depending on the temperature. I also learned something from the iPhysicsLabs column in the same issue about the oscillations in pressure (easily perceptible to passengers’ ears) as waves are reflected from the far end of a tunnel shortly after a train enters it. The April 2016 issue has a convincing experimental demonstration that the sound produced when a camera flash is discharged next to a cymbal is due to thermal expansion of the metal upon absorption of the light, and is not due to momentum transfer from the reflected photons. In the same issue, Mallmann has another of his “surprising facts” articles. His second fact asks how far apart two earths would have to be to experience the same gravitational force as two ping-pong balls in contact? His third fact shows that the circle of least confusion for light focused by a lens of large diameter is at a substantially different position than the focal plane, thus explaining a common systematic error in introductory lab measurements of focal lengths of lenses.

An article on page 113 of the February 2016 issue of the American Journal of Physics analyzes videos of a uniform thin rod as it tips over starting from a vertical position to see whether the bottom end slips or not, and if so in which direction. There is also an interesting exchange of letters on pages 146 and 147 of the same issue concerning how torques are transmitted along a rigid pivoted rod connecting two point masses located at different radii from the hinge. I am interested in the role that eddy currents play in damping the tumbling of small metal-frame satellites (such as CubeSats) in earth orbit; an article on page 181 of the March 2016 issue investigates this issue computationally and experimentally. On a similar note, the comparison between experiment and an analytic solution for a spherical magnet falling through a copper pipe on page 257 of the April 2016 issue is convincing. I also appreciated the study of supercooling of water samples of varying degrees of purity on page 293 of the same issue.

Article 015022 in the January 2016 issue of the European Journal of Physics gives an introductory derivation of the wave and beam equations starting from discrete ball-and-spring or accordion-like bending-slab models, respectively. Article 025301 in the March 2016 issue discusses how Snell’s law fails to describe the propagation of light in a medium where the surfaces of constant refractive index are not a set of parallel planes. Also, article 035602 in the May 2016 issue considers the idea (first mentioned by Feynman) that, because of gravitational time dilation, the center of the earth is a few years younger than its surface. Article 015005 in the January 2016 issue of Physics Education asks whether the 8-minute propagation time for light from the sun to reach the earth implies that the sun will already be 2° below the horizon when you see it on the horizon at sunset? Article 015010 in the same issue presents a simple approximate derivation of the temperature of a black hole. Both journals can be accessed online.

Page 213 of the March 2016 issue of Resonance has a well-written review of the theory and detection of gravitational waves. Another interesting review is of the physics of “singing” sand dunes on page 339 of the April 2016 issue.

An article on page 2094 of the December 2015 issue of the Journal of Chemical Education explains why combustion always yields about 418 kJ per mole of oxygen gas. A commentary on page 583 of the April 2016 issue proposes modifying the vague term “amount” of a substance with the adjective “stoichiometric” to make it clear one is referring to number of moles and not to mass or volume.

Article 010128 in Physical Review Physics Education Research recommends that Faraday’s law be taught by presenting (via worksheets or clicker questions) to students contrasting cases so that they discover the key features of a situation required to obtain a nonzero induced voltage or current.

The December 2015 issue of The Physics Teacher has an interesting article on page 518 explaining that giraffes drink not by repeatedly filling their mouths with water and raising their heads, nor by siphoning water up from their mouths to their stomachs like drinking straws, but rather by pumping water into their esophagus through two valves. On page 550 of the same issue there is a useful analysis of the key role played by the motional emf generated in the familiar demonstration of a current-carrying wire jumping out of a magnetic field.

An article on page 787 of the September 2015 issue of the American Journal of Physics provides an introductory-level derivation and discussion of the Boltzmann function from a theoretical, computational, and experimental point of view. I plan to try some of their arguments out this Spring in my introductory majors course. I also appreciated the analysis on page 21 of the January 2016 issue of the world’s simplest electric train YouTube video.

Article 065044 in the November 2015 issue of the European Journal of Physics points out a flaw in Galileo’s thought experiment proving that objects in vacuum cannot fall with differing speeds. If they did, he asked, what would happen if two stones of differing weights were tied together: would the combination fall faster or slower than the heavier stone alone? The flaw is that Galileo’s argument neglects the tension in the string tying the stones together. Article 065047 in the same issue examines rolling friction and mechanical energy dissipation for a ball rolling down an inclined plane.

Page 1111 of the December 2015 issue of Resonance has a nice review of the effects of radiative transfer in nature such as when viewing the sun, sky, or clouds. There is a minor figure erratum for this article.

Page 1604 of the October 2015 issue of the Journal of Chemical Education discusses the coming redefinition of the kilogram in the SI system of base units in terms of the Planck constant.

Article 020137 in Physical Review Special Topics—Physics Education Research considers whether or not it is helpful for students to decompose forces into component vectors that are drawn directly on a free-body diagram. Such diagrams end up with many arrows on them, leading to opportunities for errors and confusion about the fundamental meaning of Newton’s laws.

Every month Boris Korsunsky comes up with a challenging problem for physics teachers and students. Page 446 of the October 2015 issue of The Physics Teacher has one concerning a hanging rope sliding frictionlessly around a cylindrical peg. To extend the problem, can you find the tension in the rope as a function of the angle around the peg? Can you find the normal force on a differential segment of the rope at that angle?

An article on page 506 of the June 2015 issue of the American Journal of Physics provides an accessible explanation of how electromagnetic waves propagate through a plasma starting from Maxwell’s equations, with application to radio waves interacting with the ionosphere. On page 567 of the same issue, a remapping of a charged line segment onto a circular arc is used to provide a geometric method to obtain the direction and magnitude of the electric field due to the segment. In the July 2015 issue, an article on page 621 considers what happens if a set of capacitors are initially charged and then placed in series with a battery and optionally a resistor; this configuration results in trapped charges that could make for interesting new classroom or textbook problems on capacitor circuits. Page 646 of the same issue presents the results of a PER study that shows that allowing too many tries on online homework problems encourages counterproductive tactics; the author recommends setting the maximum number of tries to five. Turning next to the August 2015 issue, an article on page 703 gives a clear exposition of radiation reaction and the resolution of associated energy paradoxes when a charged body accelerates. A short article on page 719 of the same issue uses some clever scaling laws and molecular parameters to deduce the maximum speeds of running and swimming of animals ranging in size from bacteria to elephants and whales. Finally, page 723 presents a marvelous analysis of the following puzzle: If the same amount of heat is transferred to two identical balls, one hanging from a thread and one on a tabletop, which ends up with the larger final temperature, neglecting all heat losses? The standard answer is that it is the hanging one, because the ball on the table has to convert some thermal energy into gravitational potential energy due to its thermal expansion. However, that would violate the second law because we could attach a thread to the ball on the table after its rise and then cool it so that it would rise again, thereby converting thermal energy into useful potential energy with an efficiency that can exceed the Carnot limit.

A short article on page 564 of the September 2015 issue of Physics Education presents experimental results for the nifty effect known as a chain fountain: If the end of a long chain coiled inside a beaker is dropped over the edge of the beaker, the chain in the beaker rises up in a loop above the top of the beaker. The same issue also has an interesting discussion on page 568 of the fact that if one connects ideal batteries in parallel with each other and a resistive load, then the currents through the individual batteries cannot be determined. Turning to the European Journal of Physics, Bringuier has a remarkable analysis of damping by entropic forces in article 055024 of the September 2015 issue. In addition, article 055033 about experimentally verifying the Sackur-Tetrode equation caught my eye. Both journals are accessible online.

There are five platonic solids. If we draw a straight line passing through any vertex of such a solid and the geometric center of the solid, that line will intersect another vertex of the solid in 4 out of these 5 solids. Which is the exception and what does that have to do with the challenge question related to the cover picture of the February 2015 issue of The Physics Teacher? Also be sure to check out the incredible photograph of a balanced tower of rocks on the back page of the March issue!

An article on page 110 of the February 2015 issue of the American Journal of Physics considers a remarkable variation on Newton's cradle. Suppose a traditional cradle has 3 equal-mass balls. If ball 1 is pulled aside and released, ball 3 will come off the end of the chain with the incident velocity of ball 1. That happens because ball 1 makes 1 elastic collision with ball 2 transferring all its momentum to it, and likewise ball 2 makes 1 collision with ball 3. One can instead reduce the mass of ball 2 to sqrt(5)-2 = 23.6% of balls 1 and 3. The cradle will now function as before. This time it happens because ball 1 makes 2 elastic collisions with ball 2, to end up at rest, and likewise ball 2 makes 2 collisions with ball 3. The paper similarly finds the "magic mass ratio" of 11.0% for Newton's cradle to operate where ball 2 makes 3 elastic collisions with each of balls 1 and 3. A general formula is found for any integer number of collisions with each end ball. See a quicktime movie linked to Fig. 6 of the paper. Next, Bertrand's theorem states that only forces that vary linearly or with the inverse square of the distance from a point can yield closed orbits. Proofs of this theorem are generally not simple. However, an elementary derivation is provided on page 320 of the April issue. Incidentally, the book review of "A Student's Guide to Entropy" on page 383 of the same issue motivated me to buy that well-written text. Finally, Craig Bohren's article on page 443 of the May issue makes it clear that the difference in thermal conductivities is not the only reason that aluminum feels colder to the touch than does paper in a Veritasium video.

Page 329 of the May 2015 issue of Physics Education has a detailed study of the sliding motion of a ladder leaning against a wall, where friction with both the floor and wall is considered. Video analysis is used to compare the theory with experiment, including the loss of contact between the ladder and wall at a sufficiently small angle of inclination. Also be sure to read about David Featonby's surprising experiment showing the difference in behavior of a spinning top on an inclined plane coated with sandpaper depending on whether the base of the top is rounded or pointed on page 391 of the same issue. Turning to the European Journal of Physics, article 028001 in the March 2015 issue calculates the length of a heavy cable spanning two fixed points that minimizes the tension at the highest end. Article 035027 in the May issue uses high-speed cameras to investigate the flickering of incandescent and fluorescent lamps. Both journals are accessible online.

Page 643 of the April 2015 issue of the Journal of Chemical Education asks what produces the thick white fog seen when dry ice is dropped into water? The answer is not condensation of atmospheric water vapor onto the cold CO2 gas subliming away.

Article 010101 in Physical Review Special Topics - Physics Education Research compares student performance in adding and subtracting vectors graphically versus algebraically. The authors conclude that optimal learning occurs if students are introduced to both methods concurrently and that it is important for students to practice the graphical approach for arrows drawn in a variety of different orientations.

In the November 2014 issue of The Physics Teacher, Michael O'Shea argues that the maximum backpack weight a person can carry does not simply equal a percentage of one's body weight. Rather, there is some intermediate hiker's weight at which the backpack mass is maximized, because a person has to carry not just the pack but also their own upper body. The author's photographs of mountain hiking reminded me fondly of my own month-long Outward Bound trek in central BC (a long time ago). Hewitt's solution to Figuring Physics on page 564 of the December 2014 issue seems slightly wrong to me. He says the weight of a can of air and of an evacuated can would be equal. My issue is that should only be true if the evacuated can collapsed down to zero internal volume, which by inspection is not true for the partly collapsed can in his sketch. I invite some reader to actually try the experiment and see if they can measure a difference in weights. Page 34 of the January 2015 issue has a lovely example of the numerical simulation of the motion of a charge in a nonuniform magnetic field (namely that of an infinite straight wire) as an interesting counterpoint to the standard textbook example of helical motion in a uniform field. The authors do a nice job of deriving the two coupled differential equations. Although they cannot be solved analytically, they do have a simple dimensionless form which I find appealing.

I have previously seen granite sphere fountains, such as the one shown in Fig. 1 of the article by Snoeijer and van der Weele in the November 2014 issue of the American Journal of Physics, but I did not realize they are actually levitating a one-ton stone on a thin film of flowing water. The authors convincingly show that the levitation is due to lubrication (like a giant ball bearing) not buoyancy. In the December 2014 issue, Garfinkle and Rojo explain why meteors impact Earth with speeds that are always in the range 16 to 72 km/s. I have published a simplified analysis of the same issue. Finally, Kagan has a thorough review of the method of analyzing electrical circuits using nodal potentials (rather than the standard introductory physics method of branch currents) in the January 2015 issue. I have summarized his key example of a Wheatstone bridge of resistors for use in my classes.

Page 693 of the November 2014 issue of Physics Education presents a simple model of drafting of one race car closely following behind another. The idea is that if the front car totally cancels quadratic air drag on the second car (because they essentially form one longer car of the same cross-sectional area) then we expect the terminal speed to increase by the square root of 2, i.e., by as much as 40%. Also I was amused by the simple but clear explanation of why a superball thrown under and bouncing off the underside of a table (after hitting and returning to the floor) will retrace its path back to the thrower on pages 125 and 126 of the January 2015 issue. Turning to the European Journal of Physics, article 065012 in the November 2014 issue discusses anharmonicity in various oscillating systems: a disk bouncing between two bumpers on a horizontal air table, a marble rolling along a V-shaped track, an interrupted pendulum, a quartic rather than the usual parabolic potential, and the combination of linear damping with Hooke's law. Article 015005 in the January 2015 issue analyzes the normal modes of a vertically hanging chain with a point mass at its lower end. The addition of the mass enables Neumann functions as solutions, rather than just the Bessel functions one gets in its absence. Both journals are accessible online.

Page 2195 of the December 2014 issue of the Journal of Chemical Education discusses the thermodynamics of a rubber band, including a comparison of the theoretical equation of state with experimental results on the tension as a function of temperature and length. The references include a link to a YouTube video of a heat engine constructed by replacing the spokes with rubber bands and heating one region with a lamp. Additional theoretical and experimental details are available in the online Supplementary Info for the article.

The Indian Academy of Sciences publishes the journal Resonance of science education. Physics articles in the November 2014 issue include a review of nonlinear ocean waves on page 1047, and a brief derivation of Wien's displacement law starting from classical action on page 1058.

The article "Birds on power lines" in the July 2014 issue of the American Journal of Physics discusses three effects related to a bird contacting a single high-voltage power line: capacitive charge transfer to the "spherical" bird even if it stands on one foot, and currents in the bird between its two feet due to its finite resistance and to the ac phase difference along the wire. The August issue has a paper on page 764 that analyzes the path that a block takes when it is given a push at some angle (relative to the straight "down" direction) on a rough inclined plane. Particularly interesting is the case when the incline is small enough that the block eventually comes to rest. Finally, the September issue has a PER paper that shows that a group of South Korean high school physics students who solved an average of 2200 physics problems each did not perform significantly better on exams than those who solved comparatively few. It is not helpful to simply grind through a large number of problems, but instead some of the students' time should be spent learning the bigger picture of physics concepts and frameworks.

Page 349 of the September 2014 issue of The Physics Teacher has a nice analysis (with convincing YouTube links) of why a heavy disk at the end of a rod is much easier to lift with one hand when the disk is spinning than when it is not. The October issue has one of my all-time favorite High School Photo Contest pictures on its cover. I only hope the young lady doing the "Orange Wave" did not choke on paint particles a few seconds after the camera snapped the portrayed shot. I thought the article about writing "Letters home as an alternative to lab reports" made a convincing case and I may try it in my own classes. William Layton surprised me on page 426 by showing that a light bulb on the input side of an isolation transformer will go out if a similar bulb on the output side is unscrewed from its socket. I was similarly amazed to read on page 428 that a tuning fork with one tine covered is louder than with it uncovered. Lastly, my whole family had a fun time constructing a closed loop from a single strip of paper that can lift a bucket of water when held by a single finger, purely by folding the strip in the manner explained on page 436.

The July 2014 issue of Physics Education has an article that experimentally tests the Bernoulli equation for water flowing out of a jug on page 436. On page 390, the familiar problem of a ladder leaning against a wall is discussed, but this time with the floor smooth and the wall rough. Benacka has developed a new way to solve the Kepler problem without changing variables to u = 1/r in the July 2014 issue of the European Journal of Physics and presents an Excel spreadsheet to calculate the motion. Interesting articles in the September issue include measurement of the electric charge on a piece of Scotch tape pulled apart from another piece of tape, a simple derivation of the Boltzmann factor, and an analysis of a catenary without using the calculus of variations. Both journals are online.

Page 1455 of the September 2014 issue of the the Journal of Chemical Education has a simple derivation of shot noise in a photodiode or photomultiplier tube using the Poisson distribution.

Mazur's group has published a study in Physical Review Special Topics - Physics Education Research about the times that students take to respond to clicker questions. Response times for correct answers are shorter than times for incorrect answers, suggesting that instructors should end the polling at around the 80% response rate to cut down on the number of random guesses biasing the overall histogram of results.

Usually light pushes objects away due to the radiation pressure. Can it also attract them, creating an optical tractor beam? An article in Physical Review Letters suggests that it might, as a result of the ac Stark shifts.

Poiseuille's law says the flow rate through a pipe is inversely proportional to the length of the pipe. In the January 2014 issue of the American Journal of Physics, Michael Nauenberg explains why the flow rate nevertheless does not diverge if the outlet pipe connected to a hole in the side of a liquid tank is made vanishingly short. The February issue presents a surprising demonstration by the Naval Postgraduate School in which a styrofoam pendulum bob is attracted to a loudspeaker emitting high-amplitude low-frequency sound, rather than being jetted away from it as in the famous Maxell cassette tape ad. In the April issue, there are articles on page 280 making interferometric measurements of the collision of a steel ball with a rod on a rolling cart, on page 301 extending the Clausius-Clapeyron equation from first to second derivatives, and on page 306 discussing the advantages of plotting pressure-volume heat engine cycles on a log-log scale.

There is always so much good stuff in The Physics Teacher that it is hard to choose, but here is just one selection from each of the past five issues. Page 58 of the January 2014 issue demonstrates by breaking a light bulb and cutting off the filament that the glass of the base can be made electrically conducting by heating it with a blowtorch. Page 122 of the February issue challenges readers to construct a stable spinning top from a single paperclip. Page 142 of the March issue experimentally demonstrates the surprising fact that the turning of a paddle wheel in a cathode ray tube no more demonstrates electron momentum than does the turning of a radiometer demonstrate photon momentum. (In both cases, the momentum transfer is drowned out by heating of the residual gas in the tubes.) On page 241 of the April issue, two Portugese educators ask why it requires more work to run on an inclined than a horizontal treadmill? (The answer is simpler in a reference frame attached to the moving belt.) Finally, on page 286 of the May issue, two educators from an institution that I took a college physics course while in high school (Mount Royal University, although in my day it was Mount Royal College) point out that the traditional explanation is wrong for why Kelvin’s estimate of Earth’s age was so far off. (Accounting for radioactive minerals would only increase his estimate by about 10% which is still way too low.)

The May 2014 issue of Physics Education considers the emf generated when a cylindrical bar magnet is dropped vertically through a flat coil on page 319. If the bar magnet is short, it can be modeled as a circular loop, whereas if it is long, it can be modeled as a solenoid, in principle permitting one to estimate the magnetic dipole moment by fitting to experimental data.

There are several notable articles on thermodynamics in the March 2014 issue of the Journal of Chemical Education, including a vacancy model for the entropy change of a thermal reservoir on page 380, and a discussion of whether one should use the system pressure or the surroundings pressure in calculating expansion work during an irreversible gas expansion on page 402.

Solving for the eigenstates and energies of a hydrogen atom is a standard problem in introductory quantum mechanics. The radial part of the wavefunction exponentially tails away to infinity. But what happens if the atoms are confined, as might be an approximation of an exciton in a quantum dot? Then we must impose the boundary condition that the radial part goes to zero at some finite radius rather than at infinity. That in turn means we need to keep the second solution of the Schrödinger equation, known as Kummer functions. Read more about this analysis on page 860 of the November 2013 issue of the American Journal of Physics. Variations on this problem are a one-dimensional simple harmonic oscillator confined between two plates off which it bounces elastically, or a point charge in a capacitor (as a model of a biased quantum well).

Page 434 of the October 2013 issue of The Physics Teacher has an article entitled "Variations on the zilch cycle." A zilch cycle has a "figure 8" shape on either a P-V or T-S graph, chosen so that the two lobes of the "8" have equal area but are traversed in opposite directions. That way, both net work and net heat are zero during a cycle. A zilch cycle is thus intermediate between a heat engine (which converts net heat input into work output) and a refrigerator (which converts work input into a net heat transfer). Readers should be alert to catch and correct a number of typos in the equations in this article, however.

Some interesting experiments using a Levitron (magnetically levitated spinning top) are reported on page 67 of the January 2014 issue of Physics Education. By setting up outrigger magnets on and off the baseplate, one can tilt the spin axis of the top from the vertical all the way to a horizontal orientation! Analogs and applications of the precession and nutation are discussed.

A common science fair project is to make a battery. But typical projects use lemons which have very limited current density. A much better battery can be constructed using plates of aluminum and copper, table salt, and Drano drain cleaner, as described on page 1341 of the October 2013 issue of the Journal of Chemical Education. In a similar vein of science fair projects, see the article on page 1353 which considers the best way to construct a "soap boat" that is driven by differences in surface tension between water and another liquid (using alcohol rather than soap), as well as the article on page 1358 which cleverly considers how to build a reasonable solar cell out of household ingredients (well... except for the ITO glass slides used as substrates).

A comprehensive review of what undergraduate physics majors might need to know about tensors can be found on page 498 of the July 2013 issue of the American Journal of Physics. It includes discussion of dummy versus free indices, orthonormal bases, div/grad/curl/Laplacian, covariant versus contravariant components, basis transformations, fields, metrics, and Christoffel symbols. In the August issue, I appreciated the article by Paul Withers on page 565 about landing spacecraft on planets, where the drag force varies with altitude in the atmosphere. Descent with and without deployment of multiple parachutes is considered.

In the September 2013 issue of The Physics Teacher, I enjoyed the new editor’s Rental Car problem in the box on page 329. (Note however that the graph should be drawn a bit more clearly: Dallas is at x=0, the parabola is symmetric about its vertex at x=250 km, and Sterlington is at x=500 km.) Page 394 of the October issue presents measurements showing that a transverse pulse travels along a hanging cable at a constant acceleration of g/2. I have thought about the same setup and discuss a modified wave equation that leads to this value of the acceleration.

One can create impressive sparks by bringing a grounded ball near the top dome of a Van de Graaff generator. An Italian trio of educators show that one can photograph such a spark through a diffraction grating to determine the atomic nitrogen and oxygen ions that compose the spark. Read about it on page 426 of the July 2013 issue of Physics Education. I also liked the experiment described in the next article on page 429. Two paddles are mounted on an arm attached to a rotary motion sensor to measure the density of air. An accurate value is found by dropping the prefactor of C/2 in the expression for quadratic air resistance, where C is the drag coefficient of the paddles. Although that neglect seems surprising, apparently it implies that C is on the order of 2 for their shape of paddles, at least if one includes the “added mass” effect of an object that is constantly scooping up new air as it rotates. (Recall that the standard formula for quadratic drag assumes an object falling at constant velocity, rather than one that oscillates or rotates.) Brown and Zürcher suggest on page 1095 of the September issue of the European Journal of Physics that the hysteresis in the length of duct tape when weights are hung from it could be a good way to introduce biology students to the stress-strain behavior of tendons. I also liked the idea of modeling real-world data, such as Usain Bolt’s measured performance in the 100-m dash, by considering both linear and quadratic air drag and the effect of wind on page 1227. Both journals can be accessed online.

As a non-chemist, I found helpful the article on page 1003 of the August 2013 issue of the Journal of Chemical Education. It describes how can one construct pyramids out of plastic Lego blocks to describe the arrangement of elements in the periodic table.

Thomas Bensky and Matthew Moelter outline an introductory-level computer analysis of the kinematics and dynamics of a bead sliding on a frictionless wire on page 165 of the March 2013 issue of the American Journal of Physics. Art Hobson argues that the fundamental constituents of relativistic quantum reality are fields, not particles, on page 211 of the same issue. In a short note on page 313 of the April issue, we are reminded of how important it is to keep track of which variables are being kept constant during partial differentiation; that truth is particularly important in statistical mechanics, but the presented example contrasts the partial derivative of the kinetic energy with respect to a generalized coordinate in Lagrangian and Hamiltonian dynamics. Finally, Alejandro Jenkins tells us the history of a fraudulent perpetual motion machine on page 421 of the June issue that fooled even Leibniz and Bernoulli, while Selmke and Cichos draw an instructive analogy between Rutherford and optical scattering on page 405.

The February 2013 issue of The Physics Teacher has various descriptions of useful mechanics experiments: using buoyancy to measure the volume of a helium balloon on page 93 and to consider the change in apparent weight of an immersed object on page 96, and using an Atwood machine to measure dry axle friction in the pulley. On page 155 of the March issue, Frank Wang reminds us that a moving clock may not appear slow, owing to the finite signal propagation time from the clock to the observer. Speaking of motion relative to observers, that is what is important for the force on moving charges in a magnetic field, as discussed on page 169 of the same issue. Steve Iona reviews the 50 years of publication of TPT at the beginning of the April issue, and Mikhail Kagan solves the classic fox and rabbit chase problem by an elegant use of nonorthogonal coordinates on page 215.

A short but accurate calculation of Baumgartner's velocity of fall starting from 39 km above New Mexico is found on page 139 of the March 2013 issue of Physics Education. Some class demos about surface tension using soap films are presented on page 142. Mark Harrison compares impedance matching of resistors to perfectly inelastic collisions in mechanics on page 207. Ciocca and Wang discuss why moonlight often appears silvery or bluish on page 360 of the May 2013 issue, even though spectroscopically moonlight is redder than sunlight. Also don’t miss the contrast between blowing toward a candle from behind a menu, a wine bottle, or a funnel on pages 414 and 416. David Rowland has written another paper in his series about longitudinal motion for transverse string waves on page 225 of the March 2013 issue of the European Journal of Physics. Both journals can be accessed online.

I enjoyed Howard DeVoe's contrast of the local and global formulations of thermodynamics in the May 2013 issue of the Journal of Chemical Education. Be sure not to overlook the online Supporting Information in which he performs a computer-based Eulerian integration of the equations for a pinned vertical piston subject to wet friction (with vacuum on one side and an ideal gas on the other) that is suddenly released while the cylinder is immersed in a constant-temperature fluid reservoir.

If you make the effort to correct the large number of typos, there are some interesting comparisons of the times required for objects to move vertically due to gravity along various special paths on page 398 of the September 2012 issue of the Latin-American Journal of Physics Education.

Geoff Dougherty recommends the Fulbright faculty exchange program on page 947 of the November 2012 issue of the American Journal of Physics. Do stars twinkle when viewed from Mars? Find out at the end of the article on page 980 of the same issue. I also appreciated the detailed book review of Mark Levi's (nb: not Mark Live's) book "Why Cats Land on Their Feet" on page 1112 of the December 2012 issue of AJP, together with an online link to a further list of comments on the book. I was amused by the explanation of why bubbles sink in stout beer glasses on page 88 of the February 2013 issue. (It has to do with the tilt of the sides of the glass relative to the vertical, but I think it's just an excuse to go out drinking.) Jeroen Spandaw shows that the principle of least action can be tricky to apply correctly on page 144 of the same issue: one needs to start with boundary values (two different fixed values of y) rather than initial conditions (values of y0 and v0).

Rod Cross's measurements of the viscoelastic properties of Silly Putty on page 527 of the December 2012 issue of The Physics Teacher motivated me to summarize the analysis in more detail. I was also intrigued by Göran Grimvall's brief discussion on page 530 of the same issue about presenting an unlabeled graph and several possible captions to students and asking them to figure out which is the only correct match. Finally, the article by Hester and Burris on page 534 shows that it is possible to derive the rocket thrust equation from Newton's second law if one is careful about choice of system, and Craig Bohren discusses cooling rates of humans in air and in water on page 560. In the January 2013 issue, you might be interested in the analysis of Baumgartner's supersonic balloon jump on page 14, the surprising longitudinal momentum imparted to air flow in the near-field of a sinusoidally driven loudspeaker on page 16, the question of whether a thin or a thick fuse connected in parallel across the same potential difference would burn out first on page 38, a lovely brief derivation for maximum range of a projectile fired off a tabletop on page 52 (I recommend having your students unpack some of the missing intermediate steps), and a wonderful Physics Challenge Problem on page 56 involving two Carnot refrigerators running in a tent.

Two identical balloons are filled, one with air and the other with helium, with their ends held pinched closed and then released together from rest. Which balloon will deflate most quickly? Which balloon will fly higher? How will they sound different as the gases escape? The key is the difference in their average molecular masses, as you can read about on pages 782 and 783 of the November 2012 issue of Physics Education.

Nature has reviewed the year 2012 in science. In particular, don't miss the interactive guide through key numbers and a summary of the year in review for science.

Some effective demonstrations have been presented in recent issues of The Physics Teacher. Check out the use of a water-filled balloon to discuss buoyant force on page 428 of the October 2012 issue, and the counterintuitive behavior of series versus parallel springs on page 359 of the September 2012 issue.

A compact proof that M12 must equal M21 for the mutual inductances of two coupled circuits is provided by Dake Wang on page 840 of the September 2012 issue of the American Journal of Physics. I also enjoyed learning about the paradox of a floating candle that does not get extinguished on page 657 of the August 2012 issue, and the energy efficiency of the various systems in a car on page 588 of the July 2012 issue. The discussion on page 519 of the June 2012 issue of why shear is omitted from our standard discussions of divergence, gradient, and curl also intrigued me.

Connect a smaller spherical balloon to a larger one with a tube. It is not always the case that the smaller one will get smaller and the bigger one get bigger. See the experimental results and discussion on page 392 of the July 2012 issue of Physics Education. An aluminum soda can pull tab is floated on the surface of a glass of water. An electrostatically charged rod is brought near the tab. Does the tab move toward the rod, away from it, or stay still? See page 644 of the September 2012 issue for the surprising answer. Also check out the discussion of the volume of conical glasses and oval spoons on pages 502 to 504 of the July 2012 issue: half full is nowhere near half height! The journal can be accessed online.

The same webpage also gives a link to the European Journal of Physics. On page 1111 of the September 2012 issue, there is a discussion of particles sliding off the surfaces of arbitrarily shaped surfaces (not necessarily hemispherical as in the usual textbook case), with or without kinetic friction. Also see the discussion of the classic problem of whether or not one should run in the rain to keep as dry as possible on page 1321, and an analysis of the tumbling toast problem on page 1407 of the same issue. Finally, there is a good review of theoretical models and comparison to experimental measurements for falling U-shaped or piled-up chain systems on page 1007 of the July 2012 issue.

The June 2012 issue of the Journal of Chemical Education has a short but provocative discussion of how to define elements in contrast to compounds on page 832, and an article about how lightsticks work on page 910. Peter Loyson argues on page 1095 of the August issue that Galileo did not invent the Galilean thermometer. I was also sufficiently intrigued by the discussion of the laws of thermodynamics applied to open systems on page 968 of the July issue to write my own analysis.

Giovanni Organtini's model of a transistor as a flush toilet on page 221 of the April 2012 issue of The Physics Teacher is cute. The May 2012 issue of the American Journal of Physics has a theme on astronomy with many helpful articles for non-astrophysicists like myself: An explanation on page 376 of why the expansion of the universe does not result in the expansion of the size of atoms and other bound systems; a discussion on page 539 of a better way to explain star colors than using Wien's law; and a short but provocative calculation on page 417 that uses the uncertainty principle to estimate the time it takes for a pencil balanced on point to fall over.

On page 197 of the March 2012 issue of Physics Education, a nice explanation is provided of why modeling radioactivity by throwing 1000 dice and removing the ones that shows a 6 leads to a half-life that is systematically low compared to theory. The problem is that radioactive nuclei decay continuously while the dice "decay" in discrete steps as they are thrown. Other nice papers in the same issue include the discussion on page 152 of inverting a partly filled cup of water covered with a card and observing that the water does not all spill out, and a quantitative analysis on page 169 of a hanging rope slipping around a frictionless peg in terms of an Atwood machine with variable masses. The journal can be accessed online.

The same webpage also gives a link to the European Journal of Physics. On page 439 of the March 2012 issue, the melting ice-cube puzzle is discussed: When an ice cube floating in a glass of water melts, what happens to the water level in the glass? A previous publication shows the answer is it rises if we include the loss of buoyant force on the part of the ice that was above the water line. But this new paper shows the answer is it drops if we also include the thermal contraction due to the heat required to melt the ice. On page 467 of the May 2012 issue, a nice discussion appears about how to avoid artificial infinities when calculating the electrostatic potential of an infinite line charge.

There is a vigorous debate in the letters in the April 2012 issue of the Journal of Chemical Education about whether it's time to retire the model of hybrid atomic orbitals.

I appreciated Chunfei Li's model for how the bowing of a track down which a cart is rolling will lead to an apparent violation of conservation of mechanical energy in the March 2012 issue of the Latin-American Journal of Physics Education.

One of the reasons I love reading educational physics articles is that they often force me to rethink familiar physics explanations for phenomena. An excellent example is the article by Héctor Riveros on page 52 of 2012 Issue 2 of the relatively new European Journal of Physics Education. He challenges explanations for three common demonstrations: that when the end of a ruler protruding from under a sheet of newspaper at the edge of a table is given a sharp blow it breaks because of air pressure on the sheet; that water rises into an inverted jar covering a burning candle in a tray of water because the candle consumes the available oxygen; and that a stream of water is attracted to an electrically charged balloon because water is polar.

Harvey Leff has begun a five-part series of articles on demystifying entropy on page 28 of the January 2012 issue of The Physics Teacher. In addition, two articles particularly caught my eye in the December 2011 issue. Craig Bohren discusses convective and radiative cooling in "Why do objects cool more rapidly in water than in still air?" He points out that a person can survive a long time in still air at 7oC but not in water at the same temperature. Secondly, on page 567, an experiment is performed to measure the slipping angle of a ladder leaning against a wall when there is friction at both the floor and wall, unlike the usual textbook case that assumes a smooth wall. I have been interested in this same problem, but it is the clever technique used here to keep the two ends of the ladder perpendicular to the floor and wall that validates the theory.

Under the editorship of David Jackson, the American Journal of Physics is continuing to publish a diverse range of interesting problems. Check out Behroozi's article in the Nov. 2011 issue experimentally relating the internal pressure of soap bubbles to their radius and surface tension. In the Dec. issue, learn about video measurements of the added mass of a thrown beach ball, arising from the fact that the moving ball must accelerate some air around it. There is also a fabulous brain twister involving a falling block that is connected by a string and pulley to a second block sliding on a frictionless horizontal track. On page 24 of the Jan. 2012 issue are measurements of the muzzle velocity of a compressed air cannon; it turns out that primary disagreement with simple theory is not because of friction as the ball moves down the barrel, but because of the pressure drop as air flows through the valve between the pressure tank and the gun. Finally, I found Corti's note on the Gibbs paradox in the Feb. issue to be particularly enlightening in understanding the tricky issue of distinguishability of ideal gas particles.

Most of us are probably familiar with the idea that one can tell the Earth is round because the mast of a ship progressively sinks below the horizon as it sails straight away from shore. Kibble puts some numbers on a photo of a distant bridge to calculate Earth's radius on page 685 of the November 2011 issue of Physics Education.

The same webpage also gives a link to the European Journal of Physics. The November 2011 issue has an article by David Rowland and three letters by Butikov, by Burko, and by Repetto et al. discussing the surprisingly complicated matter of the correct expression for the potential energy density of a transverse wave on a string. The issue is that one needs to take correct account of longitudinal motion of string segments, which must occur if the stretching is uniform along the string. I have collected together some related discussions from the past decade of the momentum carried by mechanical waves.

The November 2011 issue of Journal of Chemical Education has an article by Fieberg and Girard suggesting a pie mnemonic for relating the various thermodynamic potentials such as enthalpy and Helmholtz free energy, along with the corresponding Maxwell relations.

Rod Cross has an article in the October 2011 issue of The Physics Teacher concerning what happens to a car when it drives off the end of a ramp. After the front wheels lose contact but the rear wheels have not, the car will begin to rotate downward about its center of mass. This has real-life implications, as a vehicle that drove off the top of a sand dune in Australia landed nose down and then rolled onto its roof, seriously injuring a passenger. Also the September 2011 article about the "magic trick" of a ring falling and getting knotted in a chain reminded me of the demonstration show at the Summer AAPT meeting about the physics of magic.

A pair of physicists ask "Is the electrostatic force between a point charge and a neutral metallic object always attractive?" in the August 2011 issue of the American Journal of Physics. Of course, they would not ask unless the answer were no, but you will have to read the article yourself for a specific worked-out example.

I teach at the U.S. Naval Academy. It's amazing how much the midshipmen love shooting stuff. Read about some video measurements that a student and a military instructor made of a potato bazooka on page 607 of the September 2011 issue of Physics Education.

The same webpage also gives a link to the European Journal of Physics. The September 2011 issue has lots of interesting articles: how a reverse sprinkler is related to a putt-putt boat and an unclamped garden hose wildly spraying around on page 1213; video evidence on page 1245 that a piece of paper placed on top of a book and dropped with it is not in free fall; a discussion on page 1293 of why it is difficult to ride a real bicycle on top of rollers; and measurements of axle friction on page 1367 for a rotating disk.

The July 2011 issue of the Journal of Chemical Education has several interesting pieces: some thermodynamic measurements of cups of water using an infrared camera on page 881; examples of using computer software to calculate propagated errors on page 916; and use of atomic units on page 921.

Finally, APS's Spotlight recently highlighted an article entitled "Strongly Modified Spontaneous Emission Rates in Diamond-Structured Photonic Crystals" in Physical Review Letters that succeeds in demonstrating a greater than one order of magnitude reduction in spontaneous emission of quantum dots embedded in a three-dimensional photonic bandgap structure.

Elisha Huggins has a three-part article on Special Relativity in the March, April, and May 2011 issues of The Physics Teacher. Also be sure to read Michael Grams's two-part discussion of whether students should be provided solutions to homework problems (and if so, from Cramster or from the textbook publisher?) to help them learn physics in the April and May issues.

I found Swendsen's article about the meaning of entropy in the April 2011 issue of the American Journal of Physics to be thought provoking. I also appreciated Lewis's Letter to the Editor in the same issue that we should start reporting the range of visible light in terms of hundreds of THz rather than only in terms of hundreds of nm.

There has been some controversy about the relative roles of gravity and of atmospheric pressure in the operation of a siphon. The May 2011 issue of Physics Education has some useful ideas and demonstrations related to this matter, such as what happens when there is a large air bubble in the siphon line or if the siphon tube runs into another wider diameter tube which runs into the reservoir. I also was intrigued by the demonstration on page 290 of the same issue about constructing a Faraday cage by enclosing a cell phone in a tin can sealed off with foil and punching a hole of increasing diameter in the foil until the phone rings.

The 1 March 2011 issue of the Latin-American Journal of Physics Education has a paper discussing the motion of a ball rolling on a spinning turntable. The trajectories are conic sections and are analogous to motions of charged particles in crossed electric and magnetic fields.

The 1 May 2011 issue of the Journal of Chemical Education has a nice discussion of using the Metropolis algorithm in an undergraduate thermodynamics course on page 574. Successive pages in the same issue have five articles that consider phase diagrams and entropy that should also be of interest to the same audience.

The online version of Physics World has an interesting discussion of the measurement of the thermal Casimir force (due to thermal fluctuations in the electromagnetic field between two objects).

The February 2011 issue of Science online has a surprising demonstration that under certain circumstances it can be easier to push a can whose top end is closed into a sand pile than an otherwise identical can open at both ends.

Many people (myself included) are interested in the physics of potato guns. Mark Denny has a very accessible analysis of how the muzzle speed and mechanical efficiency depend on the barrel length in the February 2011 issue of The Physics Teacher. I also enjoyed thinking about the "Direction of Friction" for a cylinder rolling without slipping up and down an incline in Paul Hewitt's January 2011 Figuring Physics column. Finally, if you teach thermodynamics, you may wish to consider Todd Timberlake's suggested coin-flipping activities on page 516 of the November 2010 issue.

The February 2011 issue of the American Journal of Physics has an article beginning on page 193 that compares theory and experiment for some extensions of the familiar demonstration of dropping a magnet down a conducting pipe: What happens if you drop two magnets? How does the magnetic braking force depend on the distance between the magnet and the pipe wall? In the January 2011 issue, the article "Listening to student conversations during clicker questions" gave me some interesting new ideas to help me improve my use of student response systems in introductory physics.

I enjoyed the various tidbits in the End Results section of the January 2011 issue of Physics Education. Also, the old chestnut of whether one should walk or run in the rain to minimize how wet one gets is discussed on page 355 of the March 2011 issue of the European Journal of Physics. Both journals can be accessed online.

The 1 January 2011 issue of the Journal of Chemical Education discusses the issue of dimensional analysis involving transcendental functions (such as sine or logarithm) on page 67.

I was intrigued by the discussion of how a fly can walk upside down on a ceiling without falling off in the online version of Physics World. Apparently the secret involves an emulsion of two different fluids secreted by a fly's feet.

APS recently highlighted an article in Physical Review Letters. about a state-of-the-art determination of the Avogadro constant. I find it to be an interesting exercise to brainstorm with students various ideas for how one might determine the values of such constants.

Beginning with the September 2010 issue of The Physics Teacher, one article per issue will be selected and supplemented with an interactive computer model developed using the Easy Java Simulations (EJS) code with the assistance of Wolfgang Christian. In this first issue, the article selected is "Calibration of a Horizontal Sundial" and includes three EJS models which illustrate the geometry of a north-oriented sundial's shadow for different latitudes and times of day. In the same issue, be sure to read the enlightening Letters to the Editor by John Mallinckrodt and by Eugene Mosca, reminding us that force is not equal to the derivative of momentum for a system of "variable mass."

The American Journal of Physics is also selecting one article per issue to supplement with EJS models. The October 2010 issue chose "A close examination of the motion of an adiabatic piston," which includes a link to a molecular dynamics simulation in which a box is partitioned by an insulated piston that is jostled back and forth by two different Lennard-Jones gases in the two sides of the box.

The September 2010 issue of Physics Education has a great way to demonstrate Poisson's spot in class. All you need is a laser pointer and a pin with a round head, which is much simpler than the typical setup using collimation optics and a video camera. The September issue of the European Journal of Physics discusses in "A thermal paradox" the question of which reaches a higher steady-state temperature: a thin or a thick plate of the same material uniformly illuminated on one face by a constant beam of light? Theory is compared with experimental results. Both journals can be accessed online.

A couple of articles caught my eye in the October 2010 issue of the Journal of Chemical Education. Page 1039 quantifies the hearing risk associated with exploding balloons containing hydrogen gas in class. Then on page 1071, a mechanical apparatus is discussed to model the Morse potential for anharmonic diatomic bonds.

You don't rate a chili pepper on RateMyProfessors.com? Well, maybe you or a colleague is a pizza slice or a harmonica instead! Check out the proposed new icons in the Chronicle of Higher Education.

The May 2010 issue of The Physics Teacher has a short but insightful article by Elisha Huggins about weighing a hollow cube whose walls are coated with mirrors between which a photon is bouncing. Does it matter whether the photon is bouncing vertically or horizontally? Also check out Boris Korsunsky's Physics Challenge entitled "Be There and Be Square" in the same issue. But beware because this problem is much harder than some of the ones in preceding issues!

The May 2010 issue of the Latin-American Journal of Physics Education has a lengthy article entitled the "Sliding rope paradox" which discusses a rope suspended over a frictionless peg off of which it is sliding. The connection with the well-known falling chain problem is also considered.

There have been plenty of arguments about how airplane wings create lift. The most recent article on this topic is by Silva and Soares in the May 2010 issue of Physics Education. Another well-discussed problem is that of crossing a river in a boat. O'Shea considers some complications involved in that task in an article in the July 2010 issue of the European Journal of Physics. Look for both journals at http://iopscience.iop.org/journals.

The Journal of Chemical Education has finally implemented a fully electronic submission procedure and a spiffy new webpage for accessing their journal. You might be interested in one chemistry educator's heuristic interpretation of quantum mechanics on page 559 of the May 2010 issue.

Some interesting letters to the editor appeared in the May 2010 issue of Physics Today, stimulated (excuse the pun) by the January 2010 article about the discovery of the ruby laser in 1960.

Finally, Art Hobson of the University of Arkansas passed along the following. The Jan-Feb 2010 issue of Environment has an article entitled Now is the time for action authored by 16 members of the National Science Foundation's Advisory Committee for Environmental Research and Education. They argue that "environmental issues must become a priority for the security of citizens and governments around the world," that "the world is at a crossroads" with "little time to act," and that "conducting research and education via a model of business-as-usual will not be sufficient." The Committee makes five recommendations: (1) increased support for interdisciplinary environmental research; (2) NSF must become a more interdisciplinary organization that attracts integrative research and education; (3) NSF must lead in implementing an integrated system of observational sensor networks that measure environmental variables and related human activities; (4) new approaches are needed for environmental education and public engagement; and (5) scientists must help policymakers develop a better understanding of environmental systems, including tipping points and the socio-economic effects of environmental change.

The November 2009 issue of The Physics Teacher has an unusually large number of articles with insightful physics experiments and theory, including discussion of cosmic ray detection, inverse-square forces, the Coanda effect, back emf, static equilibrium, general relativity, capillary rise, terminal velocity, the Coriolis force, and boomerangs, among many other topics.

I enjoyed the Quick Study entitled "The surprising motion of ski moguls" on page 68 of the November 2009 issue of Physics Today. It clearly explained how moguls form (with what characteristic spacing and amplitude) and why they slowly migrate uphill by about 8 cm per day. Get out your skis and test some physics in action!

A French trio present a pedagogically instructive mechanical model for a Carnot engine on page 106 of the January 2010 issue of the American Journal of Physics. The hot reservoir consists of a partially filled egg carton of balls located vertically higher than another egg carton comprising the cold reservoir. The mean and standard deviation of the gravitational potential energy and configurational entropy are easily calculated, and therefore also the effective reservoir temperatures and the engine efficiency.

Recently in our department there has been discussion of how to best experimentally investigate and model damped oscillations of a mass on a spring. So it was with great interest that I noticed an article on page 121 of the January 2010 issue of the European Journal of Physics. Three physicists from Colombia present data for a sphere oscillating in aqueous glycerin solutions of different concentrations and container sizes. They show that it is necessary to introduce corrections to the standard Stokes model in order to fit the data.

A set of MathCad symbolic mathematics programs and associated documentation for various topics of relevance to thermodynamics and statistical mechanics are included in the December 2009 issue of the Journal of Chemical Education, as can be accessed online.

David Griffiths (recently retired from Reed College) has an article with his thoughts about teaching physics entitled "Illuminating physics for students" in the September 2009 issue of Physics World.

In the October 2009 issue of The Physics Teacher, H.K. Wong points out on page 463 a flaw in a simple explanation of a unipolar motor (made of a battery, nail, rare-earth magnet, and wire) that I have often demonstrated in class. The torque which rotates the magnet cannot be due to the internal current flowing through the magnet. Instead it must arise from a reaction to the force that the magnet exerts on the wire near the point at which they touch each other.

I enjoyed Jeremy Bernstein's biographical ruminations about Dirac (and other physicists of his era) on page 979 of the November 2009 issue of the American Journal of Physics.

I find simple demonstrations of atmospheric buoyancy to be amusing and instructive. The November 2009 issue of Physics Education discusses two. On page 668, a person stands on a scale while wearing a Santa suit that can be filled with air. Does the scale reading change noticeably? On page 569, a syringe (with its tip capped off) is placed on a sensitive balance. Does its measured weight depend on whether the plunger is pressed in or pulled out? In one case the answer is no and in the other the answer is yes. If you add a volume of air to an object, both the gravitational and buoyant forces increase by the same amount, unless the added air is at a substantially different pressure than the surrounding atmosphere. (Now you're ready to try the alka-seltzer in a latex glove demo that Harold Stokes presented at an AAPT meeting a few years ago.)

A couple of papers caught my eye in the most recent two issues of European Journal of Physics. On page 1173 for September 2009, Agrawal discusses a simplified version of the Curzon-Ahlborn (CA) engine. Unlike a Carnot device which optimizes the efficiency but at the expense of infinitely slow operation, a CA engine maximizes the rate at which work is output. Secondly, using a numerical wind-tunnel model on page 1365 of the November issue, a Spanish pair of applied physicists show that bicyclists traveling as a tight group benefit not only the behind riders (by drafting) but even the cyclist at the front of the pack!

A brief overview of photoacoustic spectroscopy of nanomaterials can be found on page 1238 of the October 2009 issue of the Journal of Chemical Education. This technique is particularly appropriate for materials that scatter light too much to be easily studied by conventional absorption spectroscopy. The idea is to place a sample in an air-tight chamber, hit it with a chopped laser so that the sample and hence the surrounding air is periodically heated, and measure the resulting pressure oscillations with a microphone.

In the May 2009 issue of The Physics Teacher, Vincent Toal and Emilia Mihaylova present a two-page article entitled "Double-Glazing Interferometry." They explain how one can easily see white-light fringes by looking at the full moon against a black night sky through a double-paned window at an angle. A secondary image of the moon with interference fringes appears beside the moon. I found it also worked to look at a large street lamp, so there's no need to wait for a full moon. I must admit I have often seen such secondary images previously through my house windows at night but had not taken any notice of them, a demonstration of the fact that "discovery activities" need to be guided to be truly effective.

An article entitled "On the stability of electrostatic orbits" in the May 2009 issue of the American Journal of Physics discusses the stability of two charged conducting spheres orbiting each other in free space. Effects of charge polarization and dependence on the orbital angular momentum are analyzed. The first two references in the paper are to the actual demonstrations of such orbits using graphite-coated styrofoam spheres aboard the "Vomit Comet" aircraft by undergraduate students.

In the featured paper "A simple demonstration of a general rule for the variation of magnetic field with distance" in the May 2009 issue of Physics Education, a Japanese geophysicist discusses a simple method to measure the variation in magnitude of the field with distance along the axis of a small permanent magnet using only an ordinary compass. The idea is to position the magnet's axis to be perpendicular to earth's magnetic field so that the tangent of the compass needle's deflection angle gives the ratio of the magnet's field strength to that of the earth. The connection to the magnet's dipole moment is analyzed.

"A simple derivation of Kepler's laws without solving differential equations" in the May 2009 issue of the European Journal of Physics presents an elegant geometrical derivation of Kepler's three laws where the force of gravity is approximated as a succession of impulses (so that the orbit is an elliptically shaped polygon). The key step is to introduce the Runge-Lenz ("eccentricity") vector to obtain the equation of an ellipse in polar coordinates.

A fairly new journal, featuring pedagogical physics articles in both English and Spanish, is the Latin-American Journal of Physics Education, published in January, May, and September. For example, the May 2009 issue includes articles about laboratory determinations of Malus's law, properties of a pendulum, and Planck's constant.

From time to time, the Journal of Chemical Education has articles of interest to physics educators, particularly in the areas of thermodynamics and statistical mechanics. A notable example is the January 2009 issue whose "Research: Science and Education" section focuses on articles discussing entropy, the Clausius-Clapeyron equation, and the virial expansion.